Before running Synaptic Package Manager, you need to first run:

$ sudo apt-get update
and just let the command finish on its own.

Alternatively, you could run the Update Manager Check function only -
do not run the Install Update or you will get more than you bargained for.

Next, after doing one of the two above actions, then launch Synaptic
Package Manager, and select what you want to install.

Let us know if how you fare following this procedure with Synaptic
Package Manager.